FIELD OF THE INVENTION
This invention relates generally to ultra-large scale integration (ULSI) MOS integrated circuits. More particularly, it relates to a method for fabricating deep submicron CMOS integrated circuits with a self-aligned silicide gate electrode so as to eliminate poly-Si depletion and to suppress the penetration effects of boron ions.
As is generally well-known, a CMOS (complementary metal-oxide semiconductor) device is comprised of an N-channel MOS device and a P-channel MOS device. In particular, deep-submicron CMOS is the primary technology for ULSI (Ultra-Large Scale Integration) systems. In order to increase the speed of the MOS devices, there has existed in the microelectronics industry over the last two decades a continuing trend of scaling-down the structures to smaller and smaller sizes. However, as the device dimensions are scaled down, the gate oxide thickness has to be likewise reduced down to provide optimal device performance.
Thus, there has been proposed heretofore of using a P.sup.+ -type polycrystalline silicon (poly-Si) gate so as to provide a surface channel feature in P-channel MOS devices in deep-submicron CMOS structures. This is due to the fact that surface-channel P-channel MOS devices with P.sup.+ -type poly-Si gates can improve short-channel and sub-threshold I-V characteristics and produce better controllability of the threshold voltage. Typically, BF.sub.2.sup.+ ions are implanted simultaneously with the forming of the P.sup.+ poly-Si gate and a P.sup.+ -N shallow junction. The presence of fluorine ions during the BF.sub.2 implantation enhances the diffusion of boron ions. As a result, there will be a penetration of boron ions through the gate oxide which introduces boron ions to the underlying silicon substrate. Boron penetration results unfortunately in degrading the reliability of the devices, such as positive shifts in the threshold voltage, increased sub-threshold swing, and increased electron trapping.
Accordingly, one of the major concerns for existing dual gate CMOS technology is the problem of boron penetration due to gate oxide scaling-down. Another major concern caused by the gate oxide reduction is poly-Si gate depletion which produces an "excess oxide thickness" that can be quite significant in an ultra-thin gate oxide. The poly-Si depletion will degrade the drive current ability of the P-channel MOS devices. Therefore, the problems of boron penetration and poly-Si gate depletion are considered to be the two important factors which limit the performance of deep submicron devices.
In view of the foregoing, there still exists as need of a method for fabricating deep-submicron CMOS integrated circuits with a self-aligned silicide gate electrode so as to eliminate poly-Si depletion and to suppress the penetration effects of boron ions.